Re: Frames of Reference

[Leigh Palmer <palmer@sfu.ca>]

> Message text written by INTERNET:phys-l@atlantis.uwf.edu
> > > Does the physicist, checking for momentum conservation in a bubble
> chamber
> > > photograph, worry about the earth providing a moving frame of reference
> > > or do they operate as we do in a conventional lab, assume the lab is an
> > > inertial frame?
> >
> > He doesn't have to. The quantity which is important in collisions is the
> > impulse exchanged. These interactions take place over such a short time
> > that any impulse due to gravitational forces is utterly unobservable,
> > though the gravitational forces are in principle quite observable.
> >
> > That's a good problem to give your students, however. Many will say "The
> > gravitational force is just too small to matter". That is not the case, of
> > course. If the particles stuck around for a tenth of a second in the spark
> > chamber they would fall just as far as a tennis ball in the same time. The
> > question will test whether or not they are thinking.
> >
> > Leigh
>
>
> Do the calculation Leigh.  In bubble chambers most  particles travel
> at v > 0.99c. For a 10 m chamber I get a drop of about 10^14 m - very
> unobservable. (They hang around for about 3 x 10^-8 s.)

I miss the point (and I'm sure you missed a minus sign). Is "utterly
unobservable" different from "very unobservable"? The gravitational
force on a subatomic particle is, indeed, small compared to that which
acts upon an elephant, but it is not small in the proper scale of
things. The gravitational acceleration is the same for a subatomic
particle as it is for a falling elephant (ouch). (Has it has even been
measured directly for electrons and positrons?) The force per unit
mass is "small" to the same degree as it is "small" for the elephant.

Leigh